A styrene-based thermoplastic resin is a resin synthesized by homo-polymerizing styrene as a main raw material or co-polymerizing it with monomers to obtain advantages from properties of each monomer.
As the styrene-based resin, there are general purpose polystyrene (GPPS) resins and expendable polystyrene (EPS) resins obtained by homopolymerizing styrene, high impact polystyrene (HIPS) resins and styrene-butadiene rubber (SBR) resins being copolymers of styrene and butadiene rubber, and ABS resins obtained by grafting styrene and acrylonitrile to a butadiene rubber.
In addition, there are ASA resins obtained by co-polymerizing styrene and acrylonitrile with an acrylic rubber as a base, MBS-based impact reinforcing agents or transparent ABS resins obtained by co-polymerizing styrene and methyl methacrylate (MMA) with polybutadiene as a base, or acrylic-based impact reinforcing agents obtained by co-polymerizing methyl methacrylate (MMA) and an acrylate monomer with an acrylic rubber as a base. Styrene has an advantage in processability, butadiene has an advantage in impact resistance, and acrylonitrile has advantages in rigidity and chemical resistance.
Styrene-based thermoplastic resin compositions are utilized in various applications. Rubber reinforced styrene-based resins, in particular ABS resins are used widely in electric and electronic components, office appliances, automotive components, etc. due to their excellent mechanical properties, molding processability, and the like.
ABS resins for injection is generally prepared as pellets by kneading ABS copolymers polymerized via emulsion polymerization (acrylonitrile-butadiene-styrene, hereinafter referred to as ‘emulsion polymerized ABS’) and styrene-acrylonitrile copolymer having a weight average molecular weight of 60,000 to 180,000, polymerized via bulk polymerization (styrene-acrylonitrile, hereinafter referred to as ‘bulk polymerized SAN’) and extruding them, and then used by injecting the pellets to process in a predetermined shape.
Usually, the emulsion polymerized ABS is produced to have the content of the rubbery polymer in the range of 40 to 60% by weight. When it is used in housing articles of electric and electronic products, or toys, it is possible to produce articles by modulating an appropriate ratio of the emulsion polymerized ABS and the bulk polymerized SAN depending on applications to prepare pellets via the extrusion procedure, and then injecting molten resin into a mold using an injection machine.
In case of ABS resins used for refrigerator sheets, they are produced as molded products by a blow molding method through extrusion in a form of sheet after kneading the emulsion polymerized ABS and the bulk polymerized SAN having a glass transition temperature in the range of 95 to 110° C. and a weight average molecular weight of 80,000 to 200,000, or, if necessary, the emulsion polymerized SAN having a weight average molecular weight of 200,000 to 2,000,000, to prepare pellets via the extrusion procedure.
Meanwhile, the ABS resins for blow molding used to manufacture automotive spoilers require low melt viscosity and high heat deflection temperature. Accordingly, with the emulsion polymerized ABS, the emulsion polymerized α-methylstyrene-acrylonitrile having a weight average molecular weight in the range of 150,000 to 250,000 (AMS heat-resistant SAN) is used alone or in combination with the bulk polymerized α-methylstyrene-acrylonitrile having a weight average molecular weight in the range of 80,000 to 200,000 (AMS heat-resistant SAN) to control the melt viscosity. In order to meet the required high heat deflection temperature, N-phenylmaleimide (PMI)-based copolymers (PMI heat-resistant SAN) is used in combination. Especially, α-methylstyrene-acrylonitrile copolymer having a weight average molecular weight of 150,000 or more is produced by emulsion polymerization, because it is very difficult to be produced by bulk polymerization due to its high melt viscosity.
A process for preparing automotive spoilers is to produce molded products by a blow molding method after preparing parisons through extrusion. It is possible to prepare ABS resins for blow molding to be used for manufacturing the automotive spoilers with only the existing bulk polymerized heat-resistant SAN having a weight average molecular weight of about 80,000 to 200,000. In this case, the ABS for blow molding has such low melt viscosity that parison sag can occur during the formation of the parison and uneven wall thickness of the molded product can become severe when blowing the parison. Moreover, due to degradation of chemical resistance while painting the spoiler product after molding, a cracking phenomenon occurs in a thin portion of the molded products over time during use after mounting on a car to cause spoiler crack defects.